Polythiol compounds or polyol compounds, and polyisocyanate compounds are widely used as the raw materials for urethane-based optical members. Among polyisocyanate compounds, aromatic polyisocyanate compounds are produced inexpensively and in large amounts, and can contribute to the improvement of refractive indices, and thus are preferable as raw materials for urethane-based optical members for which specifically high refractive indices are required. Specifically, 4,4′-diphenylmethane diisocyanate (hereinafter also referred to as “4,4′-MDI”) is useful as a raw material monomer for optical members having high refractive indices, from the viewpoints that they are easily available in industries, and that refractive indices are easily improved. For example, Patent Literature 1 describes that a thiol compound having a specific structure and an aromatic polyisocyanate compound are used as raw materials as a polymerizable composition for use in an optical resin that can impart a high refractive index, and 4,4′-MDI is used in Examples and Comparative Examples (Examples 6 and 7).
However, 4,4′-MDI is a compound rich in reactivity, and for example, a part of the isocyanate groups reacts with the moisture in the air to generate an amine, and the amine reacts with the isocyanate groups of other molecules to generate urea bonds, whereby a compound in which molecules are linked by urea bonds is formed. Therefore, in the case when a use amount of 4,4′-MDI is increased in accordance with increasing of a refractive index desired for an optical member to such a higher refractive index that goes beyond about 1.60, a problem that turbidity and clouding, which are deemed to be caused by the precipitation of the above-mentioned compound linked by urea bonds, easily appear in the obtained optical member, and thus the optical member is poor in transparency, was caused.
As a technique for solving such problem, Patent Literature 2 describes that an urethane-based optical member that causes no turbidity and clouding, is excellent in transparency and has a high refractive index can be obtained by putting a compound formed by linking molecules by urea bonds by mixing 4,4′-MDI into a dissolved state in a polyisocyanate compound as a raw material, reacting the obtained compound with a polythiol compound, to thereby incorporate the compound in a structure that forms an optical member.